1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device containing a Schottky barrier diode.
2. Description of the Prior Art
In the prior art for manufacturing a bipolar semiconductor device for example, it has been a common practice to form a polycrystalline or amorphous silicon layer directly under the electrode to prevent aluminum or aluminum-containing material used in the electrode from reacting with the silicon involved and short-circuiting the emitter-base and base-collector junctions. It is also a practice to use a Schottky barrier diode in such a bipolar semiconductor device, in which case, however, there must be no polycrystalline silicon layer in the metal-semiconductor contact area. In the contact area constituting a Schottky barrier, if the semiconductor is polycrystalline or amorphous, a uniform contact between the metal and the semiconductor is prevented due to varied crystal grain sizes of the semiconductor. This makes it difficult to attain the properties of the Schottky barrier diode desired for its proper operation with good reproducibility. Thus, an improved manufacturing process is required.
FIGS. 1 to 5 each are a cross-sectional view of a portion of the bipolar semiconductor device mentioned above in certain sequential stages of its manufacturing process which is useful in explaining a typical conventional method of manufacturing the same. The manufacturing method of the prior art will now be described with reference to these drawings.
Referring to FIG. 1:
(1) On a silicon semiconductor substrate 1, an n.sup.+ -type buried layer 2 is formed, followed by the epitaxial growth of an n-type silicon semiconductor layer 3, in which are formed a p-type isolated region 4 contacting the substrate 1 and a p-type base region 5. The surface is covered with a field oxide layer 6. Reference characters B, S and C denote a region in which a base, a Schottky barrier diode and a collector contact are to be formed, respectively.
(2) The field oxide layer 6 is patterned by ordinary photolithography to provide windows 6S, 6E, 6B and 6C in which a Schottky barrier diode, an emitter, a base contact and a collector contact are to be formed, respectively. The etching process is done until the field oxide layer 6 is as thin as approximately 1,000 A as shown by reference character t. Reference numeral 7 denotes a photoresist layer used as a mask in this process.
Referring to FIG. 2:
(3) A second photoresist layer 8 is formed which has a pattern for exposing the bottom of the windows 6E, 6B and 6C in which an emitter, a base contact and a collector contact are to be formed, respectively.
(4) The field oxide layer 6 is etched, with the second photoresist layer 8 serving as a mask, to expose the bottom of the windows 6E, 6B and 6C in which an emitter, a base contact and a collector contact are to be formed, respectively.
Referring to FIG. 3:
(5) The photoresist layers 7 and 8 are all removed.
(6) A polycrystalline silicon layer 11 of approximately 800 A thick is formed by the chemical vapor growth method in which monosilane SiH.sub.4 is decomposed.
(7) A new mask of photoresist film is formed, which is utilized in introducing an impurity of the conductivity opposite to that of the base region to form an emitter region 9 and a collector contact region 10.
(8) A photoresist layer 12 is formed that has an opening for exposing the top surface of the field oxide layer 6 in the window 6S in which a Schottky barrier diode is to be formed.
Referring to FIG. 4:
(9) The polycrystalline silicon layer 11 is patterned, with the photoresist layer 12 serving as a mask, to expose the top surface of the field oxide layer 6 in the window 6S.
Referring to FIG. 5:
(10) The field oxide layer 6 is etched to complete the window 6S in which a Schottky barrier diode is to be formed. In etching the field oxide layer 6, a new photoresist layer, if necessary, is to be formed.
(11) The photoresist layer 12 is removed, and a first aluminum layer electrode is attached. Wiring and some other required work will complete the semiconductor device product.
According to the prior art, attachment of polycrystalline silicon onto the substrate surface where a Schottky barrier diode is to be formed can be avoided only when two photoresist layers are used. The photoresist material used must be of the negative type.
Generally, the positive type photoresist is more suitable for attaining microfabrication than the negative type, but if the positive type is applied in two layers, the first layer is melted when the second layer is coated thereupon. Thus this type of photoresist cannot be used in the process of the prior art.